A peptide containing 43 amino acid residues, rich in glutamic acid and lysine, was originally isolated from calf thymus and designated thymosin beta 4 [T.L.K. Low, S. -K. Hu, and A. L. Goldstein (1981) Proc. Nat. Acad. Sci. USA 78, 1162-1166]. However, thymosin beta 4 was also shown to be present in other tissues of rats and mice, with highest concentrations in spleen and peritoneal macrophages [E. Hannappel, G.-J. Xu, J. Morgan, J. Hempstead, and B.L. Horecker (1982), Proc. Nat. Acad. Sci. USA 79, 2172-2175]. We have now identified the same peptide in tissues of other mammalian species and other vertebrate classes, including birds and amphibia. Exceptions are the rabbit and bony fish, where thymosin beta 4 is replaced by different peptides, similar in size and in amino acid sequence. None of these peptides was detected in several invertebrates or in the protozoan, Tetrahymena pyriformis. In subcellular fractionation of rat spleen, thymosin beta 4 was recovered in the cytosol.

A peptide containing 43 amino acid residues, rich in glutamic acid and lysine, was originally isolated from calf thymus and designated thymosin beta 4 [T.L.K. Low, S. -K. Hu, and A. L. Goldstein (1981) Proc. Nat. Acad. Sci. USA 78, 1162-1166]. However, thymosin beta 4 was also shown to be present in other tissues of rats and mice, with highest concentrations in spleen and peritoneal macrophages [E. Hannappel, G.-J. Xu, J. Morgan, J. Hempstead, and B.L. Horecker (1982), Proc. Nat. Acad. Sci. USA 79, 2172-2175]. We have now identified the same peptide in tissues of other mammalian species and other vertebrate classes, including birds and amphibia. Exceptions are the rabbit and bony fish, where thymosin beta 4 is replaced by different peptides, similar in size and in amino acid sequence. None of these peptides was detected in several invertebrates or in the protozoan, Tetrahymena pyriformis. In subcellular fractionation of rat spleen, thymosin beta 4 was recovered in the cytosol.